The present invention relates to a method for the removal of dissolved oxygen from the aqueous media used in steam generation systems by the addition of hydrogen to react with the dissolved oxygen in contact with a catalyst.
Dissolved oxygen has been identified as a contributor in the corrosion of components in the primary and secondary coolant systems of nuclear power systems. It has been indicated that even very low levels of dissolved oxygen, even about 20 parts per billion or less, can contribute to pitting, denting and oxidation of copper alloy components of the system. There are numerous water storage tanks both in the primary and secondary systems, such as the condensate storage tank, auxillary feedtank, demineralized water storage tank, reactor makeup tank and refueling water storage that normally contain a supply of stagnant aerated water which, upon introduction of such water into the steam generation system can introduce oxygenated water thereto. Such storage tanks are often located outside a steam generation system enclosure and in cold temperatures, and there is a need to deoxygenate these reservoirs of oxygenated water.
Problems involved with the presence of dissolved oxygen in water and various methods for removal of such oxygen, both physical and chemical, are discussed in the paper, "Saving Energy by Catalytic Reduction of Oxygen in Feedwater", by F. Martinola, presented at the 41st Annual Meeting International Water Conference, at Pittsburgh, Pa. on Oct 20-22, 1980, the contents of said paper being incorporated by reference herein. The Martinola paper discusses the use of a palladium containing polystyrene-based anion exchange resin to aid in the reaction of hydrogen, added to an aqueous medium, with dissolved oxygen in the aqueous medium. In a system described therein, hydrogen is added to water, containing 8 ppm oxygen, which is passed through a mixing tank containing plexiglass rings, and the mixture then flows to a reactor which contains a palladium catalyst. The residual oxygen content of the water after passage through the catalyst column was 0.025 ppm (25 ppb). Other systems using hydrogen and the palladium catalyst gave residual oxygen contents of between about 0.015 ppm to 0.025 ppm (15-25 ppb).
Other systems have previously used hydrogen to remove dissolved oxygen from water. In U.S. Pat. No. 1,725,925, a system is described where oxygen is purged from water by gases such as hydrogen or nitrogen, the system displacing oxygen from the process flow with an inert gas. In U.S. Pat. No. 3,052,527 a method is disclosed using a catalyzed absorbent bed and a separate source of hydrogen gas within the bed which is intermittently maintained in an atmosphere of hydrogen gas. While the method described uses hydrogen to recombine oxygen in the presence of a catalyst, it uses activated carbon as a support medium to hold a palladium catalyst, and hydrogen is injected into the catalyst bed and a limited amount of oxygenated water is processed until the bed is depleted. Then the cycle is repeated, so a to provide only intermittent operation of the system. This use of a non-continuous hydrogen addition is apparently due to the limited slow ability for dissolving hydrogen. In U.S. Pat. No. 3,294,644 a method is described for reducing corrosion conditions in water-cooled reactors with hydrogen additions, with the oxygen controlled using hydrogen to promote the recombination of oxygen originating from radiolysis of water, requiring a radiation flux.
It is an object of the present invention to remove dissolved oxygen from an aqueous media to a value of less than about 10 parts per billion, using a catalyzed hydrogen reaction with the dissolved oxygen.
It is another object of the present invention to remove dissolved oxygen from a supply of water to a value of less than about 10 ppb for use in a steam generation system.
It is a further object of the present invention to remove dissolved oxygen from a supply of water contained in a storage tank, such that the supply of water can be used in a steam generation system without adverse affects on the components of the system.